- Eberron Campaign Setting (WotC). Even if you don't like Eberron, and clearly many don't, this book contains a full campaign setting, with prestige classes, core feats, and even an adventure. I recall a sense of wonder reading through it, enjoying the "Magic as Technology" setting and its cleverness. Unfortunately, most of the follow-up projects were a let down, including the fuzzy adventures. It was a take on a D&D world, but it had a very particular flavor that didn't work for everyone.
- Pathfinder: Rise of the Runelords (Paizo). I've been thumbing through this adventure series, but I've hesitated to read it only because I hoped someone would run it for me some day. One review from rpg.net reported: "If the print version of Dungeon had to end so that we could have Pathfinder, then I’d say it’s a good trade." I can tell you it hasn't been much of a success in the store, with only three customers buying each installment. Still, it makes me drool when I look at the maps and funky artwork. I think it gets extra votes from mobilized Paizo fans.
- Ptolus (Malhavoc Press). The giant, limited editon, original urban 3.0 playtest adventure by D&D 3 designer Monte Cook. What can I say, I'm a fan boy and I'm running Ptolus (my campaign starts up again tomorrow). The reason not to vote for it? One poster wrote: "It's because it's a big brick that I never read it, and its cost has deterred everyone I play with from buying a copy, which means there's been no opportunity to play it." At $120, it didn't get as much play as it would have as four $30 books. It's also out of print now, planned in advance as a single print run.
- Unearthed Arcana (WotC). For shear usefulness, this is the D&D 3.5 toolkit, with many alternative options for game play that many, many people have included in their games. For example, racial levels, paragon classes, dragonblooded characters, and much more. As a campaign supplement book, aimed at DM's, this one is probably the best.
- Player's Handbook II (WotC). Where Unearthed Arcana is a DM's toolkit on tweaking the game, The PHBII is a player based resource with over 100 new feats, new classes and campaign rules for things like reconstructing mid-level characters. It would get my vote as best accessory book, aimed at players, if it was broken down in that way.
Other books getting lots of votes: Spell Compendium, Magic Item Compendium, Draconomicon, and Midnight from Fantasy Flight. The Spell Compendium is the only one of those that I own and it's also, by far, the best selling book in this roundup.
As for usefulness for me, in order: Ptolus, Unearthed Arcana, Player's Handbook II, Book of the Righteous (Green Ronin), and Complete Book of Eldritch Might (Malhavoc).